PROBLEM TO BE SOLVED: To provide a microwave level meter with a horn antenna which can surely prevent the adhesion of a volatile matter to the inside of the horn antenna without complicating a device constitution, and a prevention method of the adhesion of the volatile matter to the horn antenna.SOLUTION: A microwave level meter 1 with a horn antenna comprises: a horn antenna 12 having an antenna part 10 which is arranged on a roof plate 3 of a storage tank 2 for storing a stored product 4 containing a volatile matter, emitting a microwave to a measurement face 4a of the stored product 4, and receiving the microwave which is reflected from the measurement face 4a, and a hollow truncated-cone shaped antenna horn part 11 for radiating the microwave which is emitted from the antenna part 10 to a single direction; a plurality of gas purge holes 16 which are formed at a plurality of points in the vicinity of an outlet of the antenna horn part 11; and a plurality of purge gas flow-in members 17 which are attached into the plurality of gas purge holes 16, make purge gases flow into the antenna horn part 11 from the gas purge holes 16, also make them collide with the holes, and store them in the holes.SELECTED DRAWING: Figure 4
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